1. Field of the Invention
The present invention relates to a semiconductor device and, more particularly, to a semiconductor device with a trench structure in which a MOSFET is arranged along the side of a trench.
2. Description of the Related Art
The demand for semiconductor devices with a trench structure is generally increasing because lower ON resistance can be achieved in comparison with semiconductor devices with a planar-type DMOSFET (double diffusion MOSFET) in which a channel region is formed in the surface of a semiconductor substrate.
FIG. 4 shows a semiconductor device with a conventional trench structure. A semiconductor device 101 has MOSFETs arranged along the sides of grooves (trenches) 120 formed in the surface of an N−-type semiconductor substrate (also including an epitaxial layer) 111. That is, an N−-type drain region 112, a P−-type base region 113, and an N+-type source region 114 are formed in that order from the rear along the sides of the trenches 120 and gate electrodes 121 are embedded within the trenches 120.
A gate oxide film 123, which is a thin silicon oxide film, is formed between the trenches 120 and the gate electrodes 121. The gate oxide film 123 extends to the surface of the semiconductor substrate 111 and, similarly to an insulation silicon oxide film 124 that will be described subsequently, a portion that will become a contact hole 128 is removed by etching. A P+-type base high concentration region 115 with a high impurity concentration that is linked to the base region 113 is formed in the surface portion of the semiconductor substrate 111 and a metal-layer source electrode 122 that contacts with the base high concentration region 115 and source region 114 is formed on the surface of the semiconductor substrate 111. Therefore, the base high concentration region 115 is in ohmic contact with the source electrode 122 and reduces the resistive constituent of the base region 113. Further, for the electrical insulation of the gate electrode 121 and source electrode 122, the insulation silicon oxide film 124 is formed on the upper surface of the gate electrode 121 and gate oxide film 123. When the insulation silicon oxide film 124 is removed by etching on the surface of the source region 114 and base high concentration region 115, the removed portion becomes a contact hole 128.
However, in the contact hole formation step for this semiconductor device 101, a suitable distance between the contact hole 128 and gate electrode 121, that is, a tolerance, is required so that, even when the alignment with the underlayers (each of the layers already formed for the semiconductor device 101) of the contact-hole formation photo mask is displaced or the finishing accuracy of the photoresist is somewhat poor, the contact hole 128 is not formed in the upper surface of the gate electrode 121. This tolerance is one factor that prevents the miniaturization of the semiconductor device 101.
Therefore, in order to remove one primary factor preventing the miniaturization, the semiconductor device shown in FIG. 5 has been proposed (Japanese Patent Application Laid Open No. 2002-280553, for example). Common elements that have substantially the same function as those shown in FIG. 4 have been assigned the same reference numerals and will not be described here. The semiconductor device 102 includes an insulation silicon oxide film 127 that is embedded within the trenches 120 instead of the aforementioned insulation silicon oxide film 124. The insulation silicon oxide film 127 is not formed on the upper surface of the semiconductor substrate 111, unlike the insulation silicon oxide film 124. Hence, a contact hole for establishing contact of the source electrode 122 to source region 114 and base high concentration region 115 need not be provided and, consequently, nor is the tolerance required. As a result, a more miniaturized trench-structure semiconductor device can be realized.
However, in the case of the semiconductor device 102, a step of removing the gate oxide film and the silicon oxide film formed by natural oxidation (native oxide film), on the surface of the semiconductor substrate 111, is required as a step prior to a metal-layer formation step in which a metal layer, which is to form the source electrode 122, is deposited by CVD, sputtering, or the like.
Thus, cleaning with hydrofluoric acid (hydrofluoric acid cleaning) or the like is performed. However, the insulation silicon oxide film 127 is the same oxide film as the gate oxide film, native oxide film, and the like, and is therefore removed at the same time by this cleaning. Therefore, a design in which the amount of removal achieved by this cleaning is estimated must be required and a management of the cleaning solution, cleaning time, and so forth, must be maintained highly accurately. That is, when highly accurate management, and so forth, is not performed, the amount of insulation silicon oxide film 127 removed increases and the thickness of the resultant insulation silicon oxide film 127 is then thinner than a predetermined value. Thus, in an extreme case, adequate insulation strength is no longer obtained between the gate electrode 121 and source electrode 122 and the capacity for controlling the invasion of potassium or sodium mobile ions or other mobile ions into the gate electrode 121 is reduced. There is therefore a risk of a decrease in the reliability of the semiconductor device.